A gate stack or multilayer structure is typically used as gate electrodes or interconnects in MOS and CMOS integrated circuits. One gate stack that has been proposed consists of layers of doped polysilicon or amorphous silicon, tungsten silicide (WSi.sub.2), and silicon dioxide (SiO2). The oxide layer is patterned and used as a hard mask to subsequently etch the underlying polysilicon and silicide layers. After the gate stack is patterned in this manner, the oxide hard mask is retained in the final gate structure. Consequently, the total height of the gate stack includes the height of the oxide mask layer, which contributes as much as 30% to the total height of the gate stack.
The oxide mask layer cannot be removed after the gate stack is patterned since its removal would also thin down the field oxide layer and thus adversely affect isolation between adjacent transistors. Because both the oxide mask and the field oxide layer are oxides, the etching selectivity between them is poor. Thinning down the field oxide layer can also cause significant damage which can result in defects in the integrated circuit.
Different ways of reducing gate-stack height have been attempted, but have not proven to be successful in terms of cost effectiveness as a result of low yield. One approach has involved the use of a BPTEOS hard mask with a special wet etch. One problem with the use of BPTEOS as a hard mask has been an attack on the field oxide layer due to the increase in the wet etch rate during the removal of the hard mask, especially at the edges of the field oxide, where the wet etch rate is especially high. A further problem in this approach is the tendency of boron and phosphorous impurities in the oxide to diffuse rapidly.
Previous proposals to avoid the use of oxide hard mask have failed to control the transistor dimensions with the required precision and suffer from poor linewidth control as a result of deposits formed on the patterned vertical sidewalls. In the larger pattern geometries, deposits of 100 to 200 Angstroms did not present serious problems; however, in the new generation of smaller pattern geometries, the resultant poor linewidth control may result in defective transistors or low yield.